The present invention relates generally to gas turbine engine rotor and stator assemblies, and, more particularly, to a means and method for reducing radial clearance between the stator and rotor due to circumferential distortions.
A conventional gas turbine engine includes a rotor having a plurality of circumferentially spaced rotor blades extending from a rotor disc and a stator assembly having a flowpath surface disposed adjacent to tips of the rotor blades which define a radial running clearance therebetween. The running clearance should be maintained as small as possible to ensure that all fluid flow is channeled through and not around the rotor blades for maximizing energy transfer between the fluid and the rotor blades.
The running clearance in a gas turbine engine changes both during transient operation of the engine and at varying steady state operations of the engine. This is due primarily to differential thermal movement, including expansion and contraction of structures in the engine as the engine is increased or decreased in power. The gas turbine engine includes numerous structures, and airflow and combustion gases of varying temperature. The application of different temperature to structures results in differential thermal movement during transient and, or steady state operation of the engine. Even when a structure is generally symmetric in construction and is exposed to a constant thermal input, machining tolerances can create variations in the thermal mass throughout the structure which in turn causes thermal gradients and differential thermal movement throughout the structure.
Unless differential thermal movement in the gas turbine engine structures is accommodated, thermal distortion and stress therefrom are generated. For example, upon increasing power of a gas turbine engine, rotor blades therein typically are heated and expand faster than a surrounding stator. This can result in the rotor blade tips rubbing against the stator unless the running clearance is initially set relatively large to avoid such rubs. Such large running clearance is undesirable since it decreases the efficiency of the engine.
The stator and rotor will continue to heat and expand until steady-state heat flux conditions are achieved. The stator will typically reach steady-state quicker than the rotor due to its lower thermal mass. Thus the running clearance may vary significantly during transient operation.
Alternatively, the blades may be allowed to intentionally rub the stator by using blades with abrasive tips to cut the stator surface round at a condition of maximum overlap between the stator and the rotor blades. However, a relatively large running clearance will then occur for all conditions of operation of the engine other than the one at which the maximum overlap occurred.
Conventional active clearance control structures are known for predeterminedly channeling a cooling fluid to a stator for minimizing the running clearance between the stator and the rotor blades. However, such systems are relatively complex and do not correct the cause of the varying running clearance.
Furthermore, the running clearance may be non-axisymmetric which requires more complex means for attempting to accommodate such non-axisymmetric running clearances as compared to axisymmetric running clearances. Even in situations where the non-axisymmetric running clearance is predeterminable, the means for accommodating such non-axisymmetric clearances, for example by using active clearance control, is relatively complex and does not necessarily effectively accommodate for such non-axisymmetric clearances, nor does it attempt to correct the cause thereof.
Furthermore, some non-axisymmetric running clearances are due to random occurrences such as a leak of a cooling fluid or a hot combustion gas on an adjacent structure which results in circumferential distortion and a non-axisymmetric running clearance. Once conventional means for accommodating such an occurrence would be to design for a worst case scenario and have a relatively large running clearance which is undesirable.